For a fixed \(\lambda /4\)  (700 nm) Al\(_2\) O\(_3\)  thickness, the modal reflectivity as a function of the aperture diameter is shown in Fig. REF (c). The reflectivity enhancement phenomenon is clearly observed in the subwavelength region, as marked by the dashed grey circle. In contrast, a minimum in reflectivity, which is close to zero, demonstrates the transmission resonance phenomenon as the aperture diameter becomes comparable to the laser wavelength [1]}, [2]}, [3]}.
